The characterization of Virgo data and its impact on gravitational-wave searches

Between 2007 and 2010 Virgo collected data in coincidence with the LIGO and GEO gravitational-wave (GW) detectors. These data have been searched for GWs emitted by cataclysmic phenomena in the universe, by non-axisymmetric rotating neutron stars or from a stochastic background in the frequency band of the detectors. The sensitivity of GW searches is limited by noise produced by the detector or its environment. It is therefore crucial to characterize the various noise sources in a GW detector. This paper reviews the Virgo detector noise sources, noise propagation, and conversion mechanisms which were identified in the three first Virgo observing runs. In many cases, these investigations allowed us to mitigate noise sources in the detector, or to selectively flag noise events and discard them from the data. We present examples from the joint LIGO-GEO-Virgo GW searches to show how well noise transients and narrow spectral lines have been identified and excluded from the Virgo data. We also discuss how detector characterization can improve the astrophysical reach of gravitational-wave searches.Comment: 50 pages, 12 figures, 5 table

Promotion de croissance du riz inoculé par une bactérie fixatrice d'azote, Burkholderia vietnamiensis, isolée d'un sol sulfaté acide du Viêt-nam

Des essais d'inoculation du riz par une souche bactérienne fixatrice d'azote, Burkholderia vietnamiensis TVV75, ont été menés en pots sur un sol sulfaté acide du Viêt-nam, en conditions extérieures. L'effet de cette bactérie a été testé à 3 niveaux d'azote : 0, 45 et 90 U. La souche utilisée avait été au préalable isolée parmi les bactéries fixatrices d'azote les plus abondantes de la rhizosphère du riz cultivé sur ce même sol, et sélectionnée pour son activité réductrice d'acétylène et son activité de promotion de croissance sur jeune plante. Dans cet essai, l'azote était bien le facteur limitant, comme le montre clairement la réponse de la culture du riz à la fertilisation azotée. Dans ces conditions, l'inoculation bactérienne a entraîné des augmentations significatives du tallage (12%) et de la hauteur des plantules (8%) du peuplement herbacé en pépinière, et un accroissement significatif de la production de matières sèches racinaires et aériennes tout au long de la culture. Cette augmentation de la biomasse végétale s'est accompagnée d'une augmentation du rendement final de 20% à la récolte, expliquée par un effet de l'inoculation sur toutes les composantes du rendement. Ces résultats ouvrent la voie à des applications agronomiques très prometteuses.Growth promotion of rice after inoculation with a nitrogen-fixing bacteria Burkholderia vietnamiensis isolated from an acid-sulfate soil in Vietnam. Inoculation of rice with TVV75, an N2-fixing strain of Burkholderia vietnamiensis, was conducted in a Vietnamese acid-sulfate soil in pot trials. The effect of this bacterial strain was tested for 3 nitrogen fertilizer rates, 0, 45 and 90 U under outdoor conditions. The inoculated strain has been isolated as a member of the dominant N2-fixing microflora in the rhizosphere of rice growing on the above-mentioned soil. It was selected because it exhibited a high acetylene reduction rate and high growth promotion on young plants. In the present experiment, nitrogen availability was the limiting factor for the growth of rice, which is clearly shown by the response of the plant to nitrogen fertilizer application in control pots. Under these conditions, bacterial inoculation resulted in significant increases in tiller number (12%) and plant height (8%) in nursery beds, and significant increases were also observed in both shoot and root dry weights, throughout the growth cycle, leading to a higher final yield (20%). These results are promising for further applications

Isolement et activité nitrogénasique de Burkholderia vietnamiensis, bactérie fixatrice d'azote associée au riz (Oryza sativa L) cultivé sur un sol sulfaté du Viêt-nam

Isolation and nitrogenase activity of Burkholderia vietnamiensis, a nitrogen-fixing bacterium associated with rice (Oryza sativa L) on a sulphate acid soil of Vietnam. To study the dominant diazotrophs associated with rice in Vietnam, young rice plants were grown on a Vietnamese sulphate acid soil in a phytotron; under these conditions, a maximum nitrogenase activity (estimated with acetylene reduction activity technique) of ca 360 nmol C2H4 h-1 plant-1, was measured on day 8. At this stage, N2-fixing bacteria were counted and isolated, using the 'spermosphere model' technique, in which exudates of an aseptically grown rice plantlet are the only C source for bacteria. The contribution of N2-fixing bacteria to the microflora (total bacteria, 6.5 x 108 colony forming units g-1 soil) was shown to be close to 40% in the initial rhizosphere soil. Twelve representative isolates were further studied and proved to be Burkholderia vietnamiensis. They exhibited a low pH-adapted nitrogenase activity. Four of them were selected for comparison with six other strains of Azospirillum lipoferum, A brasilense and Enterobacter cloacae for their nitrogenase activities in the presence of axenically grown rice plantlets. Strain TVV75 of B vietnamiensis was more efficient (1 900 nmol C2H4 tube-1 day-1) than the others, with the exception of strain MRB16 of A lipoferum (from Bangladesh). Consequently, strain TVV75 of B vietnamiensis has been retained for subsequent inoculation trials in Vietnam where yield increases were obtained.L'étude de la microflore bactérienne fixatrice d'azote dominante associée au riz a été menée sur un sol sulfaté acide du Viêt-Nam. Une première expérience a montré que l'activité nitrogénasique (estimée par mesure de l'activité réductrice d'acétylène) du système sol-plante intact était maximale après huit jours de croissance, en chambre climatisée (360 nmoles de C2H4 h-1 plante -1). Dans une deuxième expérience, le comptage et l'isolement des bactéries fixatrices d'azote les plus abondantes de la rhizosphère après 8 jours de culture, ont été réalisés à l'aide du modèle spermosphère. Dans ce système les exsudats d'une plante axénique de riz constituent l'unique source de carbone pour la croissance des bactéries. Le peuplement des bactéries fixatrices d'azote a été évalué dans le sol de départ à 40 % de la microflore totale cultivable qui atteint 6,5 108 UFC (unités formant colonies) par gramme de sol sec. Douze souches fixatrices parmi les plus abondantes ont été particulièrement étudiées. Ces souches appartiennent à la nouvelle espèce Burkholderia vietnamiensis. Elles ont une fixation d'azote adaptée aux bas pH. Le modèle spermosphère a été utilisé pour comparer l'activité nitrogénasique de quatre isolats de B vietnamiensis et de six souches de collection appartenant aux genres Azospirillum et Enterobacter, isolées d'autres sols. La souche B vietnamiensis TVV75 s'est avérée plus efficiente que toutes les autres souches testées (1 900 nmoles de C2H4 tube-1 jour-1) exception faite de la souche MRB16 d'A lipoferum (isolée au Bangladesh). La souche TVV75 de B vietnamiensis avait été retenue sur ce critère pour inoculer le riz au Viêt-nam dans des essais montrant des augmentations de récolte

Effects of Rice Seed Surface Sterilization with Hypochlorite on Inoculated Burkholderia vietnamiensis

When a combination of hydrogen peroxide and hypochlorite was used to surface sterilize rice seeds, a 10(2)- to 10(4)-fold decrease in CFU was observed during the first 15 h after inoculation of the rice rhizosphere organism Burkholderia vietnamiensis TVV75. This artifact could not be eliminated simply by rinsing the seeds, even thoroughly, with sterile distilled water. When growth resumed, a significant increase in the frequency of rifampin- and nalidixic acid-resistant mutants in the population was observed compared to the control without seeds. This phenomenon was a specific effect of hypochlorite; it was not observed with hydrogen peroxide alone. It was also not observed when the effect of hypochlorite was counteracted by sodium thiosulfate. We hypothesized that the hypochlorite used for disinfection reacted with the rice seed surface, forming a chlorine cover which was not removed by rinsing and generated mutagenic chloramines. We studied a set of rifampin- and nalidixic acid-resistant mutants obtained after seed surface sterilization. The corresponding rpoB and gyrA genes were amplified and sequenced to characterize the induced mutations. The mutations in five of seven nalidixic acid-resistant mutants and all of the rifampin-resistant mutants studied were found to correspond to single amino acid substitutions. Hypochlorite surface sterilization can thus be a source of artifacts when the initial bacterial colonization of a plant is studied